Joint Optimization on Mode Switching and Power Adaptation in Semantic-Assisted Hybrid NOMA

In this paper, a semantic-assisted hybrid non-orthogonal multiple access (NOMA) is investigated, in which a secondary far user (F-user) wishes to transmit to a common access point simultaneously with a primary near user (N-user) in the same frequency band. In order to take advantages of both semantic communication (SemCom) and traditional bit communication (BitCom), a SemCom/BitCom mode switching is proposed at the F-user, by adapting to the varying fading states of the channel. On this basis, in order to optimize the performance of F-user while providing adequate protection to the primary N-user, an optimization problem of maximizing the ergodic semantic rate of F-user is formulated, subject to an instantaneous bit rate constraint of N-user. This problem is non-convex and requires a joint design on the SemCom/BitCom mode switching and power adaptation over different fading states. To provide insights, it is first proved that the formulated problem satisfies the time-sharing condition, indicating that there exists a dual problem with zero duality gap to the original problem. A Lagrangian Sub-Gradient Algorithm is then proposed to solve the dual problem. Simulation results demonstrate that in the proposed semantic-assisted hybrid NOMA, significant performance improvement can be achieved for the F-user while meeting the instantaneous bit rate requirement of N-user. Furthermore, by adapting to the varying fading states, the proposed joint SemCom/BitCom mode switching and power adaptation scheme outperforms both BitCom and SemCom.